Has anyone of you found a solution to override the checkout process to archive the following situation?

1. The user adds products to the cart
2. The user goes to checkout where only Guest mode is available
3. The user fills out his name, family name, email and phone number
4. The user confirms his checkout with (cash on delivery)

5. I get the order in the backend and call the user
6. I put the products together phisically and deliver them to him/her

So what i want to, is to override the billing and payment process at checkout.

I am trying to archive this, because i want to create a mobile-first shop where it would be great to not have so many fields to fill out at the checkout.